The present invention relates to a flow control valve, and more particularly to a flow control valve which is capable of achieving accurate flow control over a wide flow range.
Flow control valves employing a needle valve construction are well known in the art. However over a wide flow range of the order of 0 to 100 liters/min, a needle valve is not practical in use because of its unavoidably large size compared with the sizes of the inlet and outlet pipes with which it is used. Furthermore, in a needle valve the relationship between the degree of opening and the flow rate is not linear. Thus, in order to overcome these difficulties, a hand-cock type of valve has been developed.
Practical examples of such flow control valves include pneumatic automatic control valves such diaphragm valves and butterfly valves, and motor-driven automatic control valves such as ball valves in which the degree of valve opening or the flow passage clearance is controlled. (See, for example, Japanese Unexamined Published Patent Application No. 121381/83).
However, in any conventional flow control valve, in order to increase the flow rate range over which the valve can be used, the size of the valve is unavoidably increased. Furthermore, as indicated by a and b in FIG. 3, which is a graphical representation showing low rate as a function of the degree of opening of a valve, it is difficult to make the relation between flow rate and the degree of opening linear over a wide range of flow rates. That is, if the relation is made linear in a low flow rate range b, then it is impossible to obtain a wide flow range, while, on the other hand, if the relation is made linear in a high flow range a, then it is difficult to obtain linearity for low flow rates.
Moreover, if a flow control valve having a wide flow range is used, then the control resolution is lowered, and moving the valve between fully opened and fully closed positions takes a relatively long time. If the resolution is improved, then the response characteristic is degraded.